iOS apps can access resources which are bundled into its Bundle during archive or the one that exists within its SandBox (resources downloaded programmatically and kept in document/temp folder of sandBox). Your audio files are on Desktop of your Mac and iOS app cant access them.
What should I do?
Select the entry of these items in your Xcode, delete them (remove only reference not original file)

now go back to desktop, select items and drag it to your project and select copy items if needed while copying

this will copy items to your app bundle now you can access it :)
Free of cost Advice
Thats a horrible coding approach to copy audio files (Assuming audio files are huge in size as I don't have context of your audio file sizes) to your app bundle :| this will bulk up your app size, think of more scalable approach, like downloading it on the fly from servers (lazy download, background download, or if you are streaming the m3U8 format etc etc)